Composite materials are the materials made of two or more constituent materials that include composite building materials such as cements, concrete; reinforced plastics such as fiber-reinforced polymer; ceramic composites; metal composites, etc. These composite materials are generally used for buildings, bridges and structures, such as boat hulls, race car bodies, spacecraft, etc. All these composite materials are susceptible to cracks due to external conditions. If the cracks form deep within the structure, detection is difficult and thus makes the repair normally impossible. These cracks affect the product's structural integrity. Since the damages deep inside materials are difficult to perceive and to repair in particular, the concept of an autonomic self-healing in a composite material is very much important, where the initiation of automatic repair is integral to the material.
Therefore, the research extensively focuses on the development of self-healing agents for composite structures. Self-healing of cracks in composite materials would contribute to a longer life of the same and would make the material not only more durable but also more sustainable.
The formation of microcrack is a critical problem in polymer composites which reduces the life time when the same is used in structural applications. Exposure to harsh environment leads to degradations of polymeric components and thus possibility of formation of microcracks. These microcracks significantly reduce the lifetime of the structures. In this context, the materials possessing self-healing function are ideal for long-lasting operation. Self-repairing polymers for polymer composites have attracted increasing research interests and attempts have been made to provide solutions in this field also.
U.S. Pat. No. 5,962,003 discloses a process for the preparation of polyurethane microcapsules containing monocrotophos. The process involves the usage of polyurethane as a wall material and the microencapsulation is conducted in two immiscible non aqueous phases at ambient temperatures.
U.S. Pat. No. 5,859,075 discloses a process for the manufacture of spherical polyurethane microspheres with controlled particle size and shape. The aforementioned document provides a process for the preparation of polyurethane mirospheres by directly using the corresponding monomers without the need for preparing any prepolymer and which can be carried out in a non-aqueous medium at low temperature.
U.S. Patent Publication No. 2006/0251688 discloses microcapsules containing biocide, useful for preparing coating materials such as interior and exterior paints. Microcapsules described in this invention are prepared by dispersing biocide in an aliphatic hydrocarbon medium containing nonionic polymeric stabilizer having hydrophilic and hydrophobic repeating units, diol or polyol having molecular weight in the range of 200-2000, crosslinker and a catalyst selected from amino or organometallic compounds; adding an isocyanate drop wise to this dispersion; agitating the mixture at 800-1000 rotations per minute for the first 3-5 hours at 40-50° C. and then at 12-15 hours at 20-27° C.; to permit the formation of polyurethane microcapsules; filtering and washing the microcapsules with lower aliphatic hydrocarbon and drying the microcapsules under vacuum at temperature between 20-35° C.
PCT International Publication No. 2015/074342 discloses a method for preparing an epoxy microcapsule by using polyurethane as a wall material comprise the following steps: preparing an endcapped-isocyanate polyurethane prepolymer; mixing the endcapped-isocyanate polyurethane prepolymer with epoxy resin, adding an emulsifier and water, stirring and performing an emulsification reaction to prepare an oil-in-water emulsion; heating and performing an interface reaction to form a capsule wall; and filtering, rinsing a precipitate by using water, and drying to obtain a microcapsule by using polyurethane as a wall material and using epoxy resin as a core material.
PCT International Publication No. 2014/194600 discloses an epoxy microcapsule with polyurea as a wall material, wherein the core material of the microcapsule comprises a liquid epoxy resin, and the wall material of the microcapsule is a crosslinked polyurea prepared by an interface reaction of polyisocyanate and water, the polyisocyanate comprising ternary or more than ternary polyisocyanate.
Article titled “Preparation and characterization of microcapsule containing epoxy resin and its self-healing performance of anticorrosion covering material” by LP Liao et al., published in Chinese Science Bulletin, 2011, 56 (4-5), pp 439-443, reports in situ encapsulation method for the preparation of urea-formaldehyde (UF) capsules filled with a healing agent, a mixture epoxy resin of the epoxy 711 and E-51. The capsules possess a uniform UF shell wall (4 μm average thickness). Successful self-healing has been demonstrated for anticorrosion covering materials with microcapsules.
As is evident from the foregoing, not only the microcapsule formulation but also the process of preparing the same is important when it comes to the durability such as thermal and mechanical stability and hence the associated properties and advantages when embedded in a composite structure. Therefore, the focus of research is currently on the development of composition of the capsules with durable properties to survive the mixing process and to release the healing agent as and when cracks appear in the hardened matrix.
In the light of the above, there remains a need in the art to provide a process for the preparation of self-healing microcapsules of epoxy resin which when embedded in composite structures releases the polymer forming material that can seal the cracks instantaneously.